1. Field of the Invention
This invention relates to a process for producing a polymer by ring-opening polymerization of a norbornene-type monomer, more specifically to a process for producing a polymer having a limited distribution of molecular weight by ring-opening polymerization at high yields.
2. Related Art
In recent years, attention has been paid to hydrogenated products of polymers produced by ring-opening polymerization of norbornene-type monomers such as tetracyclododecene, dicyclopentadiene (DCP), tricyclopentadiene, etc., as an optical material for use in an optical disk, optical lens, transparent film, etc. (e.g. JP-A-60-26024, JP-A-1-24826, WO80/01953, EP303,246, EP317,262). The reasons therefor are that such hydrogenated products are excellent in transparency and heat resistance and hardly susceptible to water and that they have comparatively small birefringence and excellent moldability.
The ring-opening polymerization of a norbornene-type monomer is carried out, in general, in the presence of (1) a catalyst system consisting of an organometallic compound such as an organoaluminum compound and a tungsten and/or molybdenum-based metathesis catalyst (e.g. U.S. Pat. No. 3,557,072) or (2) a catalyst system containing an organometallic compound such as an organoaluminum compound and a transition-metal compound such as titanium tetrahalide (e.g. JP-B-41-20111, DE-A-2,502,748).
However, in the case of the ring-opening polymerization using the above (1) catalyst system, the resultant polymer has a broad distribution of molecular weight and therefore has large birefringence, in spite of an advantage that the polymer can be obtained at such high yields that the residual monomer is hardly present in the reaction system when the reaction has completed.
In the case of the ring-opening polymerization using the above (2) catalyst system, the molecular weight distribution of a resulting polymer can be easily controlled. However, with a decreasing concentration of a monomer in the reaction system, the polymer formation ratio decreases markedly. Even if the polymerization is started with a high monomer concentration, the polymer formation ratio decreases as the monomer concentration decreases with the progress of the polymerization reaction. Hence, the polymer formed by the ring-opening polymerization of the present catalyst system cannot be obtained at high yields. Moreover, a large amount of an unreacted monomer remains in the reaction system when the polymerization has completed. It is very difficult to remove this remaining monomer at a polymer-isolation step. And, the unreacted monomer remaining in the polymer causes voids, a phenomenon of haze on a molded article surface called flash or silver lines called silver streaks.